This is a repeat competitive renewal application for a project currently in its 3rd year of NIH support. It concerns the design, synthesis and biology of bombesin/gastrin releasing peptide (Bn/GRP), neuromedin B (NMB), and related peptide analogues which could have therapeutic potential in many forms of cancer where they act as autocrine growth agents. Current goals are as follows: (1) Development of specific NMB agonists and antagonists. The NMB receptor has just been cloned and found to have entirely different ligand recognition requirements. Receptors are widely distributed in the CNS and nerve endings in the GI tract and are over-expressed in some HSCL tumors, alone or in conjunction with Bn/GRP receptors. Even more specific and potent NMB agonists have already been made and will be used as a basis for specific NMB antagonist design. All present Bn antagonists have little affinity for this receptor. (2) Further exploration of cyclic Bn peptides to be used as probes of agonist/antagonist conformation. We have recently found that D-Cys-Asn-Trp-Ala-Val-D-Ala-His-Leu-CysNH2 is a potent Bn agonist and that its psiCH2NH(13-14) counterpart is a pure antagonist. Amide-bridged cyclo[D-Phe-Gln-Trp-Ala-Val-Gly-His-Leu-Leu] is also an agonist with appreciable binding affinity. These will form the basis for further increases in affinity and simplified cyclic agonist and antagonist design. In related work, new approaches and synthetic routes are discussed for introducing conformational restraints for certain amino acid side- chains. These cyclic and restrained structures will be subjected to 2D NMR and molecular modelling techniques aimed at fine-tuning analogue design and elucidating receptor mechanisms. (3) Further improvement in receptor affinity and pharmacokinetic properties of short Bn/GRP receptor antagonists such as D-F5-Phe-Asn-Trp-D-Ala-Val-Gly-His-Leu-OMe. (4) Examination of phyllolitorin SARs -there is emerging evidence that phyllolitorin-preferring receptors are present in mammalian tissue. Phyllolitorin has important structural differences from both Bn/GRP and NMB. Also, we will examine analogue SARs for an unusual rat urinary bladder receptor on which pure antagonists display agonist activity. (5) Continued investigations on the anti-tumor growth effects of present and new Bn/GRP and NMB analogues in vitro and in vivo, particularly on human small cell lung carcinomas, prostate, breast and colon tumors, all ow which have now been shown to overexpress Bn/GRP and/or NMB receptors.